drm/radeon: disable native backlight control on pre-r6xx asics (v2)

drm/radeon: disable native backlight control on pre-r6xx asics (v2)

commit b7bc596ebbe0cddc97d76ef9309f64471bbf13eb upstream.

Just use the acpi interface.  That's what windows uses on this
generation and it's the only thing that seems to work reliably
on these generation parts.

You can still force the native backlight interface by setting
radeon.backlight=1

Bug:
https://bugzilla.kernel.org/show_bug.cgi?id=88501

v2: merge into above if/else block

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>